1. Field of the Invention
The invention pertains to a method and a system adapted to the exchange of information between a main base station and a set of mobile stations assembled for example in a “cluster”, a cluster and main station being connected by means of a “long-range” communications link.
It can be applied especially to resolving the problem of the general connectivity of a fleet of mobile vehicles with respect to a remote, main base station in taking utmost advantage of the dispersal and relative proximity of these vehicles, in order to improve the radio link between the fleet of vehicles and the main station. The information exchanged takes the form for example of packets having a destination address, for example IP type packets.
The term “connectivity” in the present invention designates a connection potential that pre-exists even when there is no transmission of information. Hereinafter in the description, reference is made to the “internal connectivity” of a cluster forming a group of several mobile stations connected to one another by a short-range communications means, taken as a whole, and to the “external connectivity” of this cluster with the main station through the “long-range” link.
More generally, the present invention can be applied in a system comprising at least one main station linked by a “long-range” communications link to one or more clusters grouping together several mobile stations communicating by means of “short-range” links.
2. Description of the Prior Art
Certain systems have a mobile communications architecture organized into several levels of communication, for example from a main station containing data to be disseminated up to the secondary mobile stations whose function is to execute the information related to the information held and transmitted by the main station. It is thus possible to define three levels of communication:    Level 1: communications within the mobile secondary station itself,    Level 2: short-range communications between several mobile stations grouped together in one and the same network or “cluster ”,    Level 3: long-range communications linking a cluster to a main station, in contrast with short-range communications internal to the cluster.
A communications architecture of this kind is found, for example, in a command system comprising a decision element supported by the main station:                1) the intra vehicle network conveys multiple-service information within a vehicle,        2) the inter-vehicle network enables short-range exchanges, for example exchanges in the kilometer range, between the different vehicles of one and the same group or cluster. This mode of operation may be envisaged during halts or movements with different performance characteristics in these two cases,        3) the long-range connectivity network, for example in the range of tens of kilometers, connects the cluster of vehicles to the main station. This external connectivity must be ensured in the fixed position as well as in movement, with however a service quality provided that is different in these two cases.        
FIG. 1 gives a schematic view of an architecture of this type where the vehicles 1, grouped together in a cluster 2, communicate with one another through arteries 3i, the cluster being linked to the main station 4 by means of a long-range network (RHC) 5 whose radio access center (RAC not shown in the figure) constitutes the entry point.
The idea is to make one or more main stations communicate with at least one cluster of vehicles linked to one another by an LAS (Local Area System).
In this type of architecture, each vehicle of the cluster is equipped, firstly, with a transmission means such as the inter-vehicle, wireless local area network, working at high bit rate within the cluster and, secondly, communications means comprising especially a long-range transmitter and receiver to and from the main station (not shown in the figure for reasons of clarity).
The implementation of the invention is based especially on the following assumption: the main station and the mobile inter-station network of a cluster are, broadly speaking, each considered to be an AS (Autonomous System) related to the IP network, administered by a single entity provided with an IP internal routing protocol or Interior Gateway Protocol (IPG) interfacing with the other autonomous systems by means of an EPG (Exterior Gateway Protocol) type protocol. This definition conforms to that of the IETF. It would also be possible to consider each of the two networks as sub-networks (for example OSPF or Open Shortest Path First areas) of one and the same AS IP.
In a first solution shown in FIG. 2, the two AS (or sub-networks) are connected by inter-AS (or inter-network) IP links, 6. The choice of the optimum route for the exchange of the information is set by the inter-AS (or inter sub-network) protocols, according to the standard procedures known to those skilled in the art. These procedures arbitrate on the optimal route.
However, this approach has the drawback of requiring the management of a spontaneous route instability or route flapping. Route flapping is an inevitable phenomenon and the related adjustments consume passband capacity on long-range links.
The idea of the invention relies on the novel approach which consists especially in connecting the two fields or sub-networks mentioned, namely the cluster and the main station, by an inter domain (or inter-sub-network) type link unique to the IP level. The routing then has only one route between the main station and the entire cluster. The alternating transmission paths of this route are determined and managed by the method and system according to the invention.